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<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      class Father {
        constructor(x, y) {
          this.x = x;
          this, (y = y);
        }
        sum() {
          console.log(this.x + this.y);
        }
      }
      //子类继承父类加法方法 同时！ 扩展！减法方法
      class Son extends Father {
        constructor(x, y) {
          // super 必须在子类this前面
          super(x, y);
          this.x = x;
          this.y = y;
        }
        subtract() {
          console.log(this.x - this.y);
        }
      }
      var son = new Son(6, 5);
      son.subtract();
      son.sum();
    </script>
  </body>
</html>
